SUPERPHOSPHATES.-" Superphosphates," using the term in its widest sense, consist principally of phosphatic minerals treated with sulphuric acid, for the purpose of rendering the phosphates soluble in water, and therefore easily accessible to the plant. It is not by any means certain that the phosphates would not be rendered soluble in course of time, if applied to the soil without previous treatment with sulphuric acid ; on the contrary, in some parts of Germany, are found deposits of mineral phosphates, which contain so much iron and lime in proportion to their phos phoric acid, that they cannot economically be used in making superphospliates, and these are applied to the land in a raw state. But the point aimed at is to supply material for the sustenance of the crop immediately to be grown, and it is small consolation to apply a dressing whose effect will not be seen for years. Therefore, except under such abnormal conditions as those stated, all mineral phosphates are dissolved in sulphuric acid, before application to the ground.
The many varieties of superphosphate are in accordance with the diversity of material used in their production. As all the mineral phosphates employed, and which form the bulk of this class of manure, undergo the same preparation, a description of the manufacture may be preceded by a notice of the most important species of this raw material.
Mineral Phosphates.—The value of a mineral phosphate may be adjudged in a great measure from its chemical composition. The first thing necessary is a large proportion of combined phosphoric acid, yielding, after manufacture, soluble tribasic phosphate of lime,—always the chief ingredient upon which the worth of a superphosphate is estimated. But it must not be taken for granted that the mineral containing the most phosphate of lime will yield the greatest proportion of soluble tribasie phosphate, for this is by no means certain. The presence of several other substances will greatly detract from the value of the sample. These are principally free oxide of iron, alumina, fluoride of calcium, silica, and carbonate of lime.
Free oxide of iron and alumina act detrimentally in two ways :—firstly, by absorbing and wasting a large amount of sulphuric acid ; and secondly, because auperphosphates made from minerals containing large proportions of these materials have a tendency to "go back" in standard, that is to aay, that a portion of the soluble phosphate becomes, after a time, insoluble again. It
may therefore happen that a mineral poor in phosphoric acid, and free from these compounds, will yield a manure of greater value than another mineral having more phosphoric acid, but con taminated with these deleterious ingredients. Iron existing as pyrites is not so mischievous, as it remains undissolved.
Fluoride of lime is objectionable chiefly as an absorbent and waster of sulphuric acid, while the gas evolved from it is highly unpleasant and unwholesome. The same may be said of chloride of lime. Their presence must also decrease the proportion of soluble phosphate.
Siliceous matters have no chemical effect upon the process either one way or another ; but, by adding to the weight and bulk of material, they lower the proportion of soluble phosphate.
Carbonate of lime is objectionable, when in very large proportions, from causing a waste of acid, and reducing the proportion of soluble phosphate, as any other foreign body must do ; but it possesses, on the other hand, a physical property which renders welcome its presence in moderate quantity, that is, that the carbonic acid liberated on ita conversion to sulphate of lime remains in the • mass for a time in a gaseous form, and thus produces in the finished manure a certain porosity or lightness, which is very desirable; and, at the same time, the sulphate of lime formed causes the manure to dry rapidly, which is one of the most important points to be attained in practice.
These remarks will be quite sufficient to indicate that, while taking the chemical analysis of a phosphate as a groundwork, it will be necessary to supplement this (supposing the analysis, of course, to warrant it) by actually trying the material with regard to the amount of acid it requires, and its physical and chemical conditions, when newly made, and after a lapse of time.